In photolithographic systems, there is a need to achieve a high resolution in order to resolve high-resolution patterns, such as images, lines, or spots. In a photolithographic system employed in the integrated circuit (IC) industry, light is projected onto a resist for the purpose of patterning an electronic device. Photolithographic systems have been used in the IC industry for many decades and are expected to resolve line widths of 50 nm and below in the future. Significant improvement in the resolution of photolithographic systems has been one of the most important enablers for the manufacture of high density and high-speed semiconductor IC chips.
The resolution R of a photolithographic system for a given lithographic constant k1, is given by R=k1λ/NA, where λ is the operational wavelength of the imaging light source, and the numerical aperture NA is given by the NA=n sin θ. Angle θ is the angular semi-aperture of the system, and n is the index of the material filling the space between the system and the substrate to be patterned.
There are three trends that are conventionally employed to effect resolution improvement in photolithographic technology. First, the wavelength λ has been progressively reduced from mercury G-line (436 nm) to the ArF excimer laser line (193 nm), and further to 157 nm and possibly into the extreme ultraviolet (EUV) wavelengths. Second, the implementation of resolution enhancement techniques such as phase-shifting masks and off-axis illumination have led to a reduction in the lithographic constant k1 from about 0.6 to about 0.4. Third, the numerical aperture NA has been increased from about 0.35 to about 0.8 with improvements in optical designs, manufacturing techniques, and metrology. However, these conventional techniques of improving the resolution are approaching physical and technical limits. For example, the value of NA, i.e. n sin θ, is limited by the value of n. If free-space optical systems are used, where the value of n is unity, the value of NA has an upper bound of unity.
Recently, immersion lithography has been developed which allows NA to be further increased. In immersion lithography, a substrate to be patterned is immersed in a high-index fluid or an immersion medium, such that the space between the final optical element or lens and the substrate is filled with a high-index fluid (n>1). In this way, the lens can be designed to have an NA larger than 1.
High-index fluids such as perfluoropolyether (PFPE), cyclo-octane, and de-ionized water may be used. Since the value of NA can be further increased, immersion lithography therefore offers better resolution enhancement over conventional lithography. The high-index fluid should satisfy several requirements: it should have a low absorption for the wavelength being used; its index of refraction should be reasonably high to make the index modification worth its while, and it should be chemically compatible with the photoresist on the substrate as well as the optical element and the coatings in contact with the fluid.
In certain prior art schemes of performing immersion lithography where water is used as the immersion fluid, the pH of the water is not controlled. Photoresists, particularly chemically amplified photoresists, may be contaminated by hydroxyl ions (OH−) present in the immersion fluid or water. Certain optic lens materials, such as calcium fluoride, dissolve in water to a certain extent.
The following references are related to aspects of the preferred embodiments and are herein incorporated by reference in their entirety.    [1] M. Switkes et al., “Methods and apparatus employing an index matching medium,” U.S. Patent Application Publication No. US 2002/0163629.    [2] J. S. Batchelder, “Method for optical inspection and lithography,” U.S. Pat. No. 5,900,354.    [3] K. Takahashi, “Immersion type projection exposure apparatus,” U.S. Pat. No. 5,610,683.    [4] T. R. Coyle et al., “Lithography system employing a solid immersion lens,” U.S. Pat. No. 5,121,256.    [5] J. A. Hoffnagle et al., “Liquid immersion deep-ultraviolet interferometric lithography,” J. Vacuum Science and Technology B, vol. 17, no. 6, pp. 3306-3309, 1999.    [6] M. Switkes et al., “Immersion lithography at 157 nm,” J. Vacuum Science and Technology B, vol. 19, no. 6, pp. 2353-2356, 2000.